142 HEREDITARY CHARACTERS 



8th October. It had three relapses, and only on 6th October 

 were the parasites at all numerous. 



Now, were the increased resistance of the trypanosomes 

 due to immunity acquired by the individuals and trans- 

 mitted by them to their offspring, there should be a steady 

 increase in the numbers of trypanosomes present in the 

 blood of the infected animal, due to the gradual increase in 

 immunity of the individuals, which is never lost but is 

 handed on to the offspring. This is not what happens. If, 

 however, we accept the explanation offered by the trans- 

 mission of inborn variations in the presence of a stringent 

 selection, the case is met perfectly. The individual trypano- 

 somes must vary among themselves as to their resisting 

 power to the new environment when transferred from the 

 rat to the monkey. Only those most resistant will survive, 

 and according to the results of the experiment but few were 

 able to survive. These multiplied, and the offspring varied, 

 some towards greater, some towards less resistance. Only 

 the former survived, and the same thing went on from 

 generation to generation, until the trypanosomes were 

 sufficiently numerous to cause a reaction in the monkey. 

 Selection then became more severe, and hardly any trypano- 

 somes were able to survive. The same thing happened again 

 at intervals, the culminating points in the struggle being 

 marked by the relapses in the monkey. When eventually 

 this highly resistant strain was again transferred to the more 

 favourable environment in the rat, they flourished in an 

 extraordinary manner, and were fatal to the rats in a far 

 less time than was the strain of trypanosomes that had not 

 been subjected to this selection. The most virulent strain 

 of trypanosomes was obtained from the monkey that showed 

 the greatest power of resistance. 



This explanation may be applied with equal or greater 

 success to the case of trypanosomes that have become re- 

 sistant to atoxyl. The effect of atoxyl is at first to lengthen 

 the period between the relapses, and to cause the trypano- 

 somes to be less numerous at these periods. At each relapse, 



